1. Field of the Invention
The present invention relates to a method and apparatus for changing the composition of a solvent in a solvent recovery system of a dewaxing apparatus, and in particular to a method and apparatus applicable to the solvent recovery system of an MEK (methyl ethyl ketone) dewaxing apparatus, for increasing or decreasing the concentration of MEK in a primary solvent which is initially mixed with the stock oil.
2. Description of the Related Art
Heretofore with known MEK dewaxing apparatus, wax and lubricating oil is produced by mixing a solvent mixture of MEK and toluene with stock oil, and then cooling the mixture and filtering.
In such MEK dewaxing apparatus, the different properties of the MEK and toluene are used in producing the wax and lubricating oil from the stock oil.
More specifically, the properties of MEK and toluene are such that MEK does not readily dissolve wax, while toluene readily dissolves both lubricating oil and wax. These properties are used in the production of low freezing point lubricating oil where the wax is first crystallized out from the stock oil in the cooling process and then filtered out with a filter.
The constituent proportions of the MEK and toluene are thus extremely important factors in the operation of the dewaxing apparatus.
If the proportion of MEK in the solvent is larger, the dewaxing temperature difference becomes smaller, and the lubricating oil yield is also improved. However if the MEK proportion is too great then oil separation can occur during filtering.
Normally the wax of low viscosity stock oil is n-paraffin based which is easily separated from the oil constituent, and hence the proportion of MEK is made large. With high viscosity stock oil however, the wax is mainly i-paraffin which is difficult to separate from the oil constituent, and hence the proportion of toluene is made large. Since the oil is dissolved in the toluene, the wax must then be separated out by precipitation.
Therefore under actual operation of the dewaxing apparatus, for optimum conditions as shown in FIG. 2 with for example the low viscosity stock oil such as S/L (Sumatra light type stock oil)--No. 10, 20 (SAE equivalent), or HDC (hydro-cracking type stock oil) No. 70 pale, 10 (SAE equivalent), the proportion of MEK is made approximately 65%, while with high viscosity stock oil such as HDC- No. 20 (SAE equivalent) and oils in wax de-oiling operations etc., the proportion of MEK is made approximately 55%.
With an ordinary dewaxing apparatus, since similar kinds of stock oil distilled from the low pressure distillation apparatus are processed, there is no real need to change the proportion of MEK and toluene significantly.
However, for cases where the stock oil for processing in the dewaxing apparatus covers many varieties such as; straight run S/L type stock oil distributed from the low pressure distillation apparatus, Middle East cracked stock oil obtained by low pressure distillation of the HDC bottoms, or wax stock oil, if the solvent composition remains the same, then due to the above-mentioned reasons, there is a reduction for example in yield and/or filterability depending on the type of stock oil, so that a low efficiency of the dewaxing operation is unavoidable.
It is therefore necessary to change the solvent composition to suit the stock oil to be processed. However, since the dewaxing apparatus comprises a cooling/precipitation system, a filtration system, and a recovery system, and the solvent is circulated through all these systems, then to change the solvent composition in the circulation circuit requires large scale modification of the equipment and the addition of a complex solvent composition changing apparatus. There is also the requirement for a complicated solvent composition changing operation and solvent control. Hence in addition to increased equipment costs there is also a deterioration in operability and controllability.